This invention relates to fusible ink sheets and, in particular, to fusible ink sheets for use in heat transfer printing.
A fusible ink sheet includes at least a substrate having at least a heat transfer ink disposed thereon. Suitable substrates include uniform resin substrates such as, for example, polyethyleneterephthalate resin and the like. Fusible ink sheets are used for heat transfer printing.
Heat transfer printing is widely used in fascimile machines, recorders and printers because of its many advantages. It is of the non-impact type and is quiet and inexpensive. Heat transfer printing can be accomplished using a small, lightweight apparatus. Additionally, it can be used to perform color printing.
A variety of heat transfer inks for fusible ink sheets have been proposed and are in use. A common requirement is that such inks must undergo phase changes, namely solid to liquid to solid in a short period of time when heat is applied. Since wax meets this requirement, heat transfer inks are often prepared by dispersing a coloring material, such as a pigment and/or a dye in a natural or synthetic wax composed primarily of hydrocarbons. Usually, small amounts of synthetic resin, plasticizer and dispersant are added in order to strengthen the wax and improve the adhesion between the ink and the substrate.
Because the ink is a mixture of wax and resin which softens and melts on heating, conventional fusible ink sheets are disadvantageous in that they are prone to blocking. Blocking refers to the undesirable adhesion between the ink layer and the substrate when the transfer sheet is wound with the layers disposed on top of each other as shown in FIGS. 1 and 2 when ink layer 103 contacts the reverse side of substrate 102 at elevated temperatures.
Blocking causes many problems. For example, when a thermal head is used for printing, blocking causes the ink to stick to the head and lowers transfer efficiency. When ink transfer is performed by applying an electric current to the fusible ink sheet, blocking increases the resistance thereby hindering transfer of ink to a transfer medium in an extreme case. Moreover, blocking makes it difficult to control the transfer density and tone of the transferred ink in full color printing.
Several methods for overcoming the blocking problem have been proposed. For example, blocking can be prevented by using a wax having a higher melting point than a conventional wax. A high melting point wax is less likely to cause blocking than a wax having a low melting point. However, high melting point waxes exhibit poor transfer efficiency. Therefore, in order to compensate for lower transfer efficiency, it is necessary to increase the print energy. This in turn decreases the life of the thermal or electrothermal transfer head.
Another proposed method for preventing blocking is to provide a transfer sheet with a release layer 301 as shown in FIG. 3. Use of release layer 301 does not solve the problem. The fusible ink sheets stick to each other. Additionally, the amount of wax present is reduced as a result of the presence of release layer 301. A release layer is generally between about 0.2 and 2 .mu.m thick and lowers heat transfer efficiency. Additionally, release layers can only be used in systems having thermal transfer heads and can not be used in systems having electrothermal transfer heads. Furthermore, the blocking problem is not completely eliminated.
In order to attempt to overcome these problems, heat transfer printers are being improved so that it is possible to produce printed images in full color. Some of the recommended systems use the dither method or the area gradation method. A common problem encountered when using these methods is that the optical density of an ink placed over a previously printed ink is poorer than the optical density of ink deposited on plain transfer paper.
Referring specifically to FIG. 4, when a cyan color 401 was transferred to a transfer paper 404 at varying energy levels ranging between 20/16 to 160/16 mJ/mm.sup.2, the optical density of the transferred cyan color increased proportionally to the amount of transfer energy applied. This result is shown by curve 501 in FIG. 5, which represents optical density as a function of applied transfer energy for cyan colored ink transferred to plain transfer paper. When a magenta color 402 was transferred to transfer paper 404 at an energy level of 160/16 mJ/mm.sup.2 and then cyan color 403 was superimposed on magenta color 402 at energy levels ranging between 20/16 and 160/16 mJ/mm.sup.2 as shown in FIG. 4, the optical density of the superimposed cyan color was significantly lower than the optical density of the cyan color transferred onto plain transfer paper 404 at corresponding energy levels. The optical density of the cyan imposed on magenta as a function of applied transfer energy is shown by curve 502 in FIG. 5.
The low optical density of the superimposed ink is a serious drawback to full color reproduction, which is achieved by superimposed magenta, cyan and yellow over each other at controlled densities. One proposed method for eliminating this drawback is to add a tackifier to the ink layer. Another proposed method is to select a solid ink having a low melting point. Both of these proposed methods improve the transfer of the second and subsequent ink layers, but they create further problems. For example, addition of a tackifier to the ink makes the ink stickier and induces blocking. The second proposed method melts the first, second and subsequent inks together so that they can be mixed and also promotes blocking.
In summary, conventional fusible ink sheets for heat transfer printing have poor blocking resistance and superimposing performance. Superimposing performance can be improved only at a sacrifice of blocking resistance. Accordingly, it is desireable to provide a fusible ink sheet having improved blocking resistance and good superimposing performance.